Have a home that is trying to improve the performance of its building shell .It has a" finished" attic that currently has R-19 batts installed on the rafters at the roof line. . This area acts as the homes interior thermal boundary but has no effective air barrier aligned with it. The side floor of the knee wall area is not insulated and there is no insulation on the attic side of the knee wall so the roof line is the clear thermal barrier .There is a small amount of duct work in the attic and i am assuming that is why the previous owner insulated the roof line, in order to bring the duct work into conditioned space.  My thought is to create an air barrier that is aligned with the insulation to limit air leakage .This seems to be a better choice instead of eliminating the existing insulation and creating a new thermal and air barrier by insulating and air sealing the side floor and insulating the knee wall.   Or the other option which would be replacing the batts and sealing off all vents and installing closed cell spray foam on the roof line which would be very expensive. Any thoughts on this?

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You don't have to think about the air sealing and insulation needs to be the same surface.  Think about air sealing the interior surface, drywall, joints, casement, base, electrical boxes.  Insulate without void.  Be careful about insulating vertical surfaces with batts.  You need to box it on all sides if the batt is not horizontal. If not the hot air just leaves the batt removing the insulation value.  This is beyond just a caulk gun. You should be sealing wire penetrations, joints and air paths.

One concept you could use is to make your existing roof deck the air and thermal barrier.  You can strip the shingles off and seal the outside of the roof then create you air space for venting and put a new roof deck and surface on top of that. 

Where do you want the air barrier to be?   I try TRY T R Y to keep the all the ducts in the building.   Think about putting 4" sheet foam under the roof line - some time its quicker/cheaper.  

For storage and duct work i would like to keep it at the roof line but it is not essential to do so , but you are correct in saying the adding additional air barrier at the roof line would be much cheaper fix than moving the air and

thermal barrier.

We accomplish both with a single coat of our Radiant Control Coating.

 But thats not all we do for the attics.  We also coat attic AC units, plenum and steel or flex ducts too.

THE RCC CLASSROOM

Is this spray coating like the "famous" insulating paint the FTC finally went after or just a spray used to air seal before insulating?

I'd consider keeping the thermal & pressure/air boundary at the roof deck (take advantage of the relatively tight roof deck) and install a sealed/taped Intello membrane as an air barrier.  Please meake sure you add enough insulation to at least meet current IECC for your zome before the sir sealing.  Be aware - soffit vents w/o propper blocking are subject to windwashing so many times (at least in the Northeast) you're far better off removing the FGB, blocking the vents and reinsulating, air barriering (is this even a word?).  We do this all the time with great results though we NEVER use fiberglass as it is far too easy for air to move though it for our liking.

Avoid the knee wall & ceiling below - fixing them is a fool's errand as the are the legs of the triangle not the hypotanuse!

Best to work with an experienced local home performance contractor / building analyst professional to achieve great results.  Talk to their clients about RESULTS before contracting with them.  Good luck.

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